You must render video at good fps, i mean how much fps have you got while recording x how much did you speed up'd it. for example i record reps at 15 fps and speed it up by 1,7500 (in properties of clip) so i must render video at 26,25 fps as 15 x 1,7500 = 26,25, most of editors ignore this and it looks like shit tbh in their vids
Also the best will be if you set same fps in properties of project (before starting editing ofc) and while rendering, there shouldn't be any bugs
ignore all errors, my english sucks, hope you understand what i mean